In addition, neurons are the longest cell, some can be over a meter long in adult.
The largest is the oocyte and it is 35 micrometer in diameter and it can be seen you the naked eye if you have good sight.

Officially the smallest bacteria are Mycoplasmas, and these bacteria are 150 nm. This is smaller than some viruses. The biggest bacteria is Thiomargarita namibiensis, it can grow to about 1 millimeter and can be seen by with the naked eye.